Normally, compressed air brake systems in commercial motor vehicles operate at a maximum system pressure of approximately 7 to 8 bar.
It has been found that the restricted amount of space which is necessary for accommodating the reservoirs and brake cylinder volumes in buses and other commercial vehicles, as well as increased usage of compressed air requirements for other ancillary equipment, are some of the reasons for developing brake systems that operate at higher pressures than previous braking systems.
Further, in the design of pneumatically-operable fully-lined disc brakes, it is necessary for the purpose of providing sufficient contact between the rotors and the brake discs. Further, it is advantageous to utilize a high pressure system since greater clamping forces can be exerted by the brake pads.
Aside from the advantages achieved through a high pressure brake system, such as smaller volume of the reservoirs, which results in quicker readiness for use due to shorter filling time, and greater braking clamping forces, there are several negative characteristics. For example, a considerable problem arises primarily due to the fact that the supply lines of a compressed air brake system act as a dead volume which totally escapes into the atmosphere after a braking application. Accordingly, in a high pressure brake system having an operating pressure of 20 bar, a considerably greater volume of unused air is vented into the atmosphere than is the case with a normal pressure brake system having an operating pressure of 7 bar.